In chemical processing plants and power plants, pneumatic systems are utilized for controlling various operations. Such pneumatic systems are analogous to electrical systems and it is often desirable to convert from pressures to voltages or currents, and then back from currents or voltages to pressures so that the system can be a combined pneumatic system and electrical system for control and monitoring purposes.
In this respect, it is often desirable to effect pressure changes in a pneumatic system from a remote control room without the need for extending piping for conducting pressure levels to the control room. It is much easier to extend wire conductors from a control panel in the control room to the location of a pressure operated pneumatic actuator or valve positioner. Accordingly, an electropneumatic transducer system is needed to convert the electrical signal to a pressure signal at the location of the pressure control element. The electrical signal is connected to the control room by wire conductors which may be as much as 1000 feet or more in length. Then the electropneumatic transducer system with a pressure control element at the remote location must be operable in response to a change in an electrical signal from the control room to effect a corresponding change in pressure.
Frequently, hazardous combustible atmospheres exist in the area of the electropneumatic transducer system. Accordingly, it is desired that low electrical currents and voltages are employed to ensure a condition of Intrinsic Safety where there is insufficient energy in any of the exposed circuit elements, e.g., wire conductors that could create a spark and ignite the hazardous combustible atmosphere. Typically, this is achieved by utilizing voltages not more than 10 volts on the output terminals from the electropneumatic transducer system and output or input currents which are not more than 20 mA.
Many of the presently available electropneumatic transducer systems, although having Intrinsic Safety suitable for operation in hazardous combustible environment, are limited in their use for a variety of reasons. First of all, many are not accurate or stable and are sensitive to temperature changes and vibrations which cause the output pressure to fluctuate. Some of these systems have a limited dynamic range because of the low voltage and current restrictions placed on the system and have low loop gain. Other of these electropneumatic transducer systems are hard to adjust. In addition, many of these electropneumatic transducer systems are housed in rather large housings and the manufacturer of such systems requires complex manufacturing and assembling techniques and special equipment. As a result, such prior systems are very expensive to produce. Also, if an explosion-proof housing is needed for the electropneumatic transducer system, the cost of the system is further increased.
Recently, a signal converting unit intended for use in a pneumatic control system has been proposed which is more accurate than many of the previously proposed electropneumatic transducer systems. Such signal converting unit is disclosed in a Patent Cooperation Treaty application filed at the Swedish Patent Office by Saab-Scania entitled A Signal Converting Unit Intended To Be Incorporated In A Pneumatic Control System, under PCT Ser. No. W0 80/01826, and published on Sept. 4, 1980 in English.
This signal converting unit receives control signals for controlling a pressure output. The pressure output is generated from a pressure source which is coupled to a nozzle and to the pressure output line. A movable piezoelectric element, flap or tongue is positioned adjacent the nozzle orifice for controlling the amount of pressurized air that is allowed to escape from the nozzle. The closer the piezoelectric element to the nozzle, the greater the output pressure, and the farther away the piezoelectric element, the lower the pressure, since more air is allowed to escape from the nozzle which reduces the pressure output. The output pressure is supplied to a pressure transducer that produces a feedback signal, a signal which can be conditioned or amplified so as to be directly related to the output pressure. This feedback signal is then compared with an input control signal, and if there is a difference between them, the comparator sends an output signal to an integrator circuit which controls the voltage applied to the piezoelectric element to cause the piezoelectric element to be moved toward or away from the nozzle orifice. In this way, the output pressure follows and is directly related to the input control signal.
As will be described in greater detail hereinafter, the electropneumatic transducer system of the present invention differs from the previously proposed system having a feedback control loop by providing:
(a) for operation of the electrical circuit for the system about a low reference and bias voltage between rail voltages of the system whereby a railsplitting circuit arrangement can be constructed with low input voltages to each circuit;
(b) a high current shutoff circuit and a low current shutoff circuit to provide hard turn-ons of the control nozzle of the system and hard turn-offs of the control nozzle at the upper and lower ends of the pressure range;
(c) a pressure transducer having piezoresistances in a Wheatstone bridge and a highly accurate zero and span temperature correction circuit;
(d) a number of open circuit contacts into which jumpers can be inserted and removed for adapting the electropneumatic transducer system for many different modes of operation such as reversal of the input/output modes of operation;
(e) a nonlinearity correction circuit coupled to the pressure transducer of the system; and
(f) circuitry for providing Galvanic isolation of the voltage sensitive nozzle flapper of the system.